Stents are generally designed as tubular support structures that can be used in a variety of medical procedures to treat blockages, occlusions, narrowing ailments and other problems that restrict flow through body vessels. Numerous vessels throughout the vascular system, including peripheral arteries, such as the carotid, brachial, renal, iliac and femoral arteries, and other body vessels, may benefit from treatment by stents. Self-expanding stents, which deploy automatically when released from an intraluminal delivery device, are often fabricated from superelastic materials such as equiatomic or near-equiatomic nickel-titanium alloys (e.g., Nitinol).
A limiting factor in many stent designs is durability. For example, stents employed in the superficial femoral artery (SFA) may be exposed to significant axial, torsional and bending stresses. In addition, due to blood flow through the vessel, stents may experience pulsatile loading on the order of 400 million cycles over 10 years of in vivo use. Fatigue life is thus a critical consideration for stent design.
Although the fatigue mechanics of nickel-titanium alloys are quite complicated, it is generally accepted that surface flaws (cracks) are initiating sites for fatigue failure. It is believed that fatigue crack growth rates in nickel-titanium alloys are higher than crack growth rates in other titanium alloys used in medical devices. Small surface cracks in a stent structure have the propensity under some loading conditions to propagate until the structural integrity of the stent is compromised.
Consequently, the surfaces of nickel-titanium alloy stents are generally highly electropolished in an effort to mitigate the impact of surface flaws on device performance. Electropolishing may not be effective in eliminating all surface flaws, however. Fatigue life improvement remains a challenge for nickel-titanium alloy stent design.